A large family of space launch vehicles—the Thor and Delta rockets—were derived from the Thor design. The Delta II is still in active service as of 2014 and with the retirement of Atlas and Titan in the mid-2000s, is the last surviving "heritage" launch vehicle in the US fleet (being derived from a Cold War-era missile system).

Fearful that the Soviet Union would deploy a long-range ballistic missile before the U.S., in January 1956 the USAF began developing the Thor, a 1,500 miles (2,400 km) intermediate-range ballistic missile. The program proceeded quickly, and within three years of inception the first of 20 Royal Air Force Thor squadrons became operational in the UK. The UK deployment carried the codename 'Project Emily'. One of the advantages of the design was that, unlike the Jupiter IRBM, the Thor could be carried by the USAF's cargo aircraft of the time, which made its deployment more rapid. The launch facilities were not transportable, and had to be built on site. The Thor was a stop-gap measure, and once the first generation of ICBMs based in the US became operational, Thor missiles were quickly retired. The last of the missiles was withdrawn from operational alert in 1963.

A small number of Thors, converted to "Thrust Augmented Delta" launchers, remained operational in the anti-satellite missile role as Program 437 until April 1975. These missiles were based on Johnston Island in the Pacific Ocean and had the ability to destroy satellites in low Earth orbit. With prior warning of an impending launch, they could destroy a Soviet spy satellite soon after orbital insertion. These missiles remain in storage, and could be reactivated, though the W-49 Mod 6 warheads were all dismantled by June 1976.

Development of the Thor was initiated by the USAF in 1954. The goal was a missile system that could deliver a nuclear warhead over a distance of 1,150 to 2,300 miles (1,850 to 3,700 km) with a CEP of 2 miles (3.2 km). This range would allow Moscow to be hit from a launch site in the UK.

Propulsion provided by half of the Navaho-derived Atlas booster engine (due, largely, to the lack of any alternatives at this early date)

10,000 mph (4.5 km/s) maximum speed during warhead reentry

Inertial guidance system with radio backup (for low susceptibility to enemy disruption)

On November 30, 1955 three companies were given one week to bid on the project: Douglas, Lockheed, and North American Aviation. They were asked to create "a management team that could pull together existing technology, skills, abilities, and techniques in 'an unprecedented time.'" On December 27, 1955 Douglas was awarded the prime contract for the airframe and integration. The Rocketdyne division of North American Aviation was awarded the engine contract, AC Spark Plug the primary inertial guidance system, Bell Labs the backup radio guidance system, and General Electric the nose cone/reentry vehicle.

Douglas further refined the design by choosing bolted tank bulkheads (as opposed to the initially suggested welded ones) and a tapered fuel tank for improved aerodynamics. The engine was developed as a direct descendant of the Atlas MA-3 booster engine. Changes involved removal of one thrust chamber and a rerouting of the plumbing to allow the engine to fit within the smaller Thor boat-tail. Engine tests were being performed as of March 1956. The first engineering model engine was available in June, followed by the first flight engine in September. Engine development was complicated by serious turbopump problems. Early Thor engines suffered from "bearing walking", where the turbopump bearings shift axially within their housing, causing rapid wear and bearing seizure.

Thor test launches were to be from LC-17 at Cape Canaveral Missile Annex. The development schedule was so compressed that plans for the Atlas bunker were used to allow the completion of the facility in time. Nevertheless pad LC-17B was just ready for the first test flight.

The first flight-ready Thor, Missile 101, arrived at Cape Canaveral in October 1956. It was erected on LC-17B and launched 25 January 1957. The Thor failed almost immediately at liftoff as the engine lost thrust, dropped back onto the pad, and exploded. Engineers could not determine the cause until viewing film of prelaunch preparations that showed crews dragging a LOX filler hose through a sandy area. It was concluded that debris had entered the LOX and contaminated it, causing valve failure.

Thor 102 was launched on 20 April. The booster was performing normally, but an erroneous console readout caused the Range Safety Officer to believe that it was headed inland and he initiated the destruct sequence 30 seconds into the launch.

The third Thor launch (Missile 103) did not get off the pad. During prelaunch preparations on 22 May, a stuck valve caused the LOX tank to overpressurize and explode, once again necessitating repairs to LC-17B.

Missile 104, launched 22 August from the newly opened LC-17A, broke up at T+92 seconds due to a drop in signal strength from the programmer, causing the engine to gimbal hard right. The guidance system tried to compensate, but ended up producing uncontrolled yaw maneuvers that caused excessive structural loads.

Thor vehicle 105 (20 September), 21 months after the start of construction, flew 1,100 miles (1,800 km) downrange. No telemetry equipment was included on this missile and the weight savings allowed it to achieve a total range of 1,500 miles (2,400 km).

Missile 107 (3 October) fell back onto LC-17A and exploded at launch due to the gas generator valve failing to open.

Missile 108 (11 October), exploded around T+140 seconds without prior warning. Engineers were bewildered as to the cause of the failure. After the first Thor-Able launch failed six months later due to a seized turbopump, it was concluded to be the cause of 108's demise, although the missile did not have sufficient instrumentation to determine the exact nature of the failure.

The final three Thor tests during 1957 were all successful. 1958 began with back-to-back failures. Thor 114 was destroyed by Range Safety 150 seconds into launch when the guidance system lost power and Thor 120's engine shut down slightly under two minutes after liftoff.

On April 19, Missile 121 dropped back onto LC-17A and exploded, putting the pad out of action for three months. A fuel duct collapse was believed to have been the culprit.

The Jupiter, Thor, and Atlas missiles all used a variant of the Rocketdyne LR-79 engine and all three suffered launch failures due to a marginal turbopump design which resulted in the bearings coming loose and causing the pump to seize (the first indication of trouble came during static firings of LR-79s in mid-1957). In February 1958, Rocketdyne proposed modifying the bearing retainers, but the Air Force's Ballistic Missile Division ignored this suggestion on the grounds that there was insufficient data regarding the turbopumps' performance. Meanwhile, the Army Ballistic Missile Agency (in charge of the Jupiter and Redstone programs) conducted a series of laboratory tests at Huntsville, Alabama in which it was determined that the decrease in air pressure at high altitudes caused lubricating oil in the bearings to foam, resulting in their failure. Modifications to the existing stock of Jupiter missiles proved successful and none were lost to turbopump failures again.

General Bernard Schreiver, head of the Air Force Ballistic Missile Division (BMD), rejected the idea of sending Thor and Atlas missiles back to the factory and decided that he would only allow in-field modifications so as to not delay the testing program. Six consecutive Thor and Atlas launches failed during February–April 1958, although not all of them could be attributed to turbopump problems. Later in the year, Thor-Able 1 failed in-fight while performing the first attempted launch of an American lunar probe on 17 August, followed by Atlas 6B in September. After this, the Air Force gave in and agreed to replace the turbopumps in all of their missiles, after which there were no launch failures due to a turbopump problem. The necessary modifications to the missiles would have taken only one month and not caused any delay to either Thor-Able 1 or Atlas 6B's flights, thus those failures were ultimately attributed to poor management of the programs.

Five successful Thor tests were conducted in June-July 1958, the last one carrying a mouse named Wickie on a biological mission, although the capsule sank into the ocean and could not be recovered. Thor 126 (July 26) lost thrust 50 seconds into launch when a LOX valve inadvertently closed. The vehicle pitched down and broke up from aerodynamic loads.

Phase II testing with the AC Spark Plug inertial guidance system began 7 December with the first successful flight on 19 December 1957.[1]

The operational variant of the Thor, the DM-18A, began testing in the autumn of 1958, but Missile 138 (November 5) went out of control shortly after liftoff and had to be destroyed. Nonetheless, Thor was declared operational and testing now began at Vandenberg Air Force Base on the West Coast when Missile 151 flew successfully on December 16. On December 30, a near repeat performance of the November 5 failure happened when Missile 149 lost control and was destroyed 40 seconds into launch.

After a run of successful launches during the first half of 1959, Missile 191 suffered another control malfunction while being launched from VAFB. This time, the missile's pitch and roll program failed to activate and it simply continued flying straight up. Launch crews initially did nothing as they reasoned that the Earth's rotation would gradually take it away from land and they wished to continue collecting data as long as possible. Eventually though, they became nervous about it exploding or pitching over, so the destruct command was sent around 50 seconds into launch. High-altitude wind caused some debris to land in the town of Orcutt near the base. This failure was traced to a safety wire that had been meant to prevent the control tape in the programmer from inadvertently coming loose during vehicle assembly. The wire would ordinarily be cut after installation of the programmer in the missile, but Douglas technicians had forgotten this important step, thus the tape could not be spooled and the pitch and roll sequence did not activate.

All 60 of the Thor missiles deployed in the UK were based at above-ground launch sites. The missiles were stored horizontally on transporter-erector trailers and covered by a retractable missile shelter. To fire the weapon, the crew used an electric motor to roll back the missile shelter (essentially a long shed mounted on steel rails), then used a powerful hydraulic launcher-erector to lift the missile to an upright position for launch. Once it was standing on the launch mount, the missile was fueled and could be fired. The entire launch sequence (from starting to roll back the missile shelter through to ignition of the rocket engine and lift-off) took approximately 15 minutes. Main engine burn time was almost 2.5 minutes, boosting the missile to a speed of 14,400 ft/s (4,400 m/s). Ten minutes into its flight the missile reached an altitude of 280 miles (450 km), close to the apogee of its elliptical flight path. At that point the reentry vehicle separated from the missile fuselage and began its descent toward the target. Total flight time from launch to target impact was approximately 18 minutes.

The Thor was initially deployed with a very blunt conical G.E. Mk 2 'heat sink' re-entry vehicle. They were later converted to the slender G.E. Mk 3 ablative RV. Both RVs contained a W-49 thermonuclear warhead with an explosive yield of 1.44 megatons.

Johnston Island Launch Emplacement One (LE1) after a Thor missile launch failure and explosion contaminated the island with Plutonium during the Operation "Bluegill Prime" nuclear test, July, 1962. The retractable missile shelter (on rails) can be seen at the rear

9 July 1962, Thor missile 195 launched a Mk4 reentry vehicle containing a W49 thermonuclear warhead to an altitude of 250 miles (400 km). The warhead detonated with a yield of 1.45 Mt of TNT (6.07 PJ). This was the Starfish Prime event of nuclear test operation Dominic-Fishbowl.

Boyes, John. The Thor IRBM: The Cuban Missile Crisis and the subsequent run-down of the Thor Force. pub: Royal Air Force Historical Society. Journal 42, May 2008. ISSN 1361 4231.

Forsyth, Kevin S. Delta: The Ultimate Thor. In Roger Launius and Dennis Jenkins (Eds.), To Reach The High Frontier: A History of U.S. Launch Vehicles. Lexington: University Press of Kentucky, 2002. ISBN 0-8131-2245-7.